1. Technical Field
The present invention relates to an optical waveguide comprising a bonded member of a core layer and cladding layers, suitable for light source module, optical interconnection, optical communication and so forth, and also to an optical information processing apparatus such as display.
2. Description of Related Art
Heretofore, mainly electric signals have taken part in information transmission over relatively short distances, typically between boards in a single electronic instrument, and between chips on a single board, in which further increase in signal speed and in density of signal wirings will be necessary in order to upgrade performances of integrated circuits. It is, however, difficult for electric signal wirings to increase the signal speed and density of the electric signal wirings, due to problems in occurrence of signal delay or generation of noise ascribable to time constant of the wirings.
Optical interconnection now attracts a public attention as one solution for these problems. The optical interconnection is applicable to various sites such as between electronic instruments, between boards in a single electronic instrument, and between chips on a single board. For the purpose of a short-distance signal transmission such as between chips for example, an optical transmission communication system can be constructed by forming, on a substrate on which the chips are mounted, an optical waveguide which is used as a transmission line for a signal-modulated laser light and so forth.
On the other hand, there is known use of the optical waveguide as a light source module of a display. For example, there is developed a head-mounted display by which a user can enjoy movie software, game, computer screen, and cinema on its own large screen, and this realizes a personal display with which the user can readily enjoy a highly realistic movie anytime anywhere, simply by wearing it like a sunglasses (see U.S. Pat. No. 5,467,104).
The head-mounted display uses red, green and blue light-emitting diodes (LEDs) as the light sources, in which the LED light lacks coherency, has a wide angle of radiation, and is difficult to be multiplexed through condensation of three colors of light. There is known a technique of producing a uniform white light by multiplexing three colors of LED light through an optical waveguide (Nikkei Electronics, Mar. 31, 2003 Issue, p.127).
There is also known an optical waveguide 183 having a structure shown in FIG. 39 (see Patent Document 1, described later).
The optical waveguide 183 is configured, as shown in FIG. 39A and FIG. 39B, so that a cladding layer 102 of 0.5 μm thick, typically composed of InP, is formed on an InP substrate 101 of a predetermined thickness composed of a semiconductor substrate or a dielectric substrate, and a core layer 106 composed of InGaAs, having a width on an incidence surface 127 side of 50 μm and a width on an emission surface 135 side of 2 μm, and also having straight inclined surfaces 155, is formed further thereon.
A cladding layer 105 of 1 μm thick, typically composed of InP, is further formed so as to cover the cladding layer 102 and the core layer 106, to thereby configure the optical waveguide 183.
As shown in FIG. 39C, when a laser 169 (any color allowable) as an exemplary light source is disposed on the incidence side of the optical waveguide 183 on the cladding layer 102, and the light beam emitted from the laser 169 enters the core layer 106 through the incidence surface 127, the light beam narrows its range of spreading and is condensed in accordance with changes in the width of the core layer 106 as indicated by a very thick line, and is then emitted through the emission surface 135 to the external.
As another conventional example, there is also known an optical waveguide 184 having a structure shown in FIG. 40 (see Patent Document 2, described later).
In this example, the optical waveguide 184 has a double-layered structure having a core layer 106 through which the light propagates and a cladding layer 143, in which the core layer 106 in a square form is configured to fill a recess provided to the top surface of the cladding layer 143 having a nearly square form. The cladding layer 143 also serves as a substrate for holding the core layer 106, and contains a light absorbing agent 122 for absorbing the light propagating through the core layer 106 and/or light scattering intercepting agent and so forth.
As a still another conventional example, there is known an optical waveguide 184 having a structure shown in FIG. 41 (see Patent document 3, described later).
This example comprises a substrate 142 mounted in an optical waveguide module package 151, an optical waveguide 185 composed of a cladding layer 143 and a core layer 106, both of which being formed in predetermined geometries on the substrate 142, an LED 141 mounted at a predetermined position on the substrate 142, an optical fiber mounting portion 150 formed on the substrate 142, a wavelength division multiplexing (WDM) filter 148 formed in contact with the substrate 142, and a photodiode mounting carrier 147 disposed behind the WDM filter 148, in which the photodiode mounting carrier 147 has a photodiode element 146, and the WDM filter 148 has a metal film 149 vacuum-evaporated thereon and a pinhole 145 formed nearly at the center of the filter 148.
In thus-configured optical waveguide 185, a part of the light emitted from the LED 141 enters the optical waveguide 185 for the LED, and propagates in the optical waveguide 185. The light propagated in the optical waveguide 185 is totally reflected on the WDM filter 148, and then enters an optical fiber transmission line (not shown) through the optical waveguide 185 for the optical fiber.
The light from the transmission line composed of the optical fiber 140 enters the optical waveguide 185 for the optical fiber from the optical fiber 140, goes through the pinhole 145 of the WDM filter 148, and is received by the photodiode element 146.
[Patent Document 1] Japanese Patent Application Publication (KOKAI) No. HEI 5-173036 (p.2, right column, L.17 to p.3, left column, L.8, FIG. 1d, FIG. 1e)
[Patent Document 2] Japanese Patent Application Publication (KOKAI) No. HEI 2-87102 (p.2, right column, L.29 to p.3, left column, L.40, FIG. 2)
                [Patent Document 3] Japanese Patent Application Publication (KOKAI) No. 2001-305365 (p.3, right column, L.34 to p.4, left column, L.22, FIG. 1).        
However, typically in the conventional example shown in FIG. 39, use of an LED (light-emitting diode) as a light source in place of the laser 169 results in entering of the emitted light from the light source not only into the core layer 106 but also into the cladding layers 102, 105, due to a diffusing tendency of the LED light unlike the laser light, and the light then propagates through them, and goes out from the sectional surfaces of the individual cladding layers on the emission surface 135 side of the core layer 106.
The emitted light from the core layer 106 is therefore undesirably mixed with the emitted light from the cladding layers 102, 105, and makes it difficult to recognize the clear emitted light from the core layer 106, and this results in increase in diameter of the beam which should be a point light source, and makes it unacceptable for use in optical communication or display.
In the conventional example shown in FIG. 40 having the light absorbing agent 122 added to the entire portion of the cladding layer 143, the light which enters the core layer 106 and propagates therethrough while being reflected on the interface with the cladding layer 143 may be worsened in terms of light propagation loss.
In addition, mixing of carbon powder or the like into the cladding layer 143 may form regions having different refractive indices at the interface between the core layer 106 and the cladding layer 143, because the light absorbing agent 122 is less likely to uniformly distribute in the cladding layer 143, and this sort of non-uniformity in the mixing may locally modify the propagation loss and so forth of the waveguide 184.
In the conventional example shown in FIG. 41, which is the module package 151 allowing the send and receive operations at the same time by making wavelengths of the sent light and received light differ from each other, any roundabout of the incident light from the LED 141 into the photodiode element 146 is causative of noise for the light in the optical fiber 140, and this degrades the receiving characteristics of the module package.
It is known that efficiency of incidence of the emitted light from the LED 141 into the optical waveguide 185 is as much as 50% in maximum, and the residual emitted light from the LED 141 not entered the optical waveguide 185 propagates as a stray light in the air or in the cladding layer 143 in adjacent to the optical waveguide 185.
The pinhole 145 of the WDM filter 148 is provided in order to prevent the stray light from entering the photodiode element 146, after transmitting through the WDM filter 148, to thereby avoid degradation of the receiving characteristics.
However, even if any effort of eliminating the propagating light (stray light) in the cladding layer 143 should be made by providing the WDM filter 148, having the pinhole 145 formed therein, at the end surface on the light emission side of the optical waveguide 185, difficulty and labor-consuming nature of alignment between the pinhole 145 and the emission surface of the core layer 106 tends to result in misalignment, and this is not only causative of degradation of the receiving characteristics, but also degrade productivity of the module package 151 using the WDM filter 148.
The WDM filter 148 can intercept wavelength region of the emitted light from the LED 141, which is a major component of the stray light, but cannot completely intercept the stray light incident at large angles. Provision of the metal film 149 composed of, for example, CrO or the like, having a light reflectivity and light absorbency, on the light incident side of the WDM filter 148 may be one possible countermeasure, but this will increase the process steps.
Provision of a groove having a predetermined width so as to be laid in a direction crossing the direction of propagation of light, and so as to be extended over the optical waveguide 185, the cladding layer 143 and the substrate 142, and insertion of a metal plate having a pinhole, configured similarly to the WDM filter 148, will be hopeful to give a similar effect with the WDM filter 148, but this discontinues the optical waveguide 185, and consequently results in degradation of light propagation performance of the core layer 106.